1. Field of the Invention
The present invention relates to toner used in an image forming apparatus such as an electrophotographic copying machine or the like for developing electrostatic images, and more particularly to toner for developing electrostatic images, which has stable frictional charge characteristics and which does not contaminate the surface of a frictional charging material such as carrier or the like.
2. Description of the Prior Art
In an image forming apparatus such as an electrophotographic copying machine or the like, a two-component developer comprising fine particle toner and frictional charging material such as magnetic carrier or the like is usually used as a developer to make visible by a dry developing process an electrostatic latent image formed on a photoconductor. When such a two-component developer is mixed together by a fixed-type mixer, the toner is charged by friction with the carrier and adheres uniformly on the surface of the carrier. Generally, the toner contains colored resin particles of 5 to 20 .mu.m in size that are prepared by dispersing a colorant into a binding resin medium. As the binding resin that forms the toner, a resin is used, for example a styrene type resin, which has desired charging and binding characteristics for fixing the colorant to the surface of a sheet made of paper or other material. The colorants used include carbon black and other organic or inorganic coloring dyes. The main components of the toner, i.e. the colorant, the binding resin, etc., have respectively intrinsic polarities and charging characteristics, but by themselves they are not enough for developing an electrostatic latent image formed on the photoconductor. Therefore, a charge control dye (hereinafter abbreviated as CCA) is generally added in a small quantity to the toner. The CCAs used for this purpose include nigrosine; monoazo dyes; metallic complexes of salicylic acid or naphthoic acid; and so forth.
The CCA is used to activate the generation of electrostatic charges in the toner and thereby to enhance the saturation charge amount thereof. It is therefore desirable that the CCA exist on the surfaces of toner particles in view of charging through friction with the frictional charging material such as the carrier or the like. However, since most CCAs are hydrophilic, it is hard to say that the CCAs have good compatibility with the binding resin, therefore they are not uniformly dispersed in the toner. Nonuniform dispersion of the CCA results in inconsistent charge characteristics of the toner. Moreover, if the CCA existing on the surfaces of toner particles falls off and adheres to the surfaces of carrier particles, they will cause the surfaces of carrier particles to become contaminated. This leads to a phenomenon during a continuous copying operation, in which, although the initial image may be produced in good condition, the charge amount of toner gradually decreases involving an increase in the image density and eventually causing such problems as dust collection on the ends of the carrier particles.
Reducing the amount of the CCA to be used may be considered as a possible solution to the problem caused by the nonuniform dispersion of the CCA and to the problem of the carrier surface contamination by the CCA. However, when the amount of the CCA to be used is reduced, the frictional charge efficiency of the toner decreases and the saturation charge amount thereof also decreases. A decreased frictional charge efficiency means increased time is required for the toner to be charged to a prescribed amount, which, in particular, causes the problem of the splashing of uncharged toner during a continuous copying operation, thus impairing the quality of the images produced.